Percutaneous transluminal coronary angioplasty (PTCA) provides an alternative to coronary artery bypass graphs or open-heart surgery as means for treating atherosclerosis. Atherosclerosis is a particular type of heart disease wherein the deposition of fatty material on the inside of vessel walls throughout the body causes the artery to narrow and restrict the flow of blood. If the artery becomes too narrow, the heart muscle that is nourished by the artery receives insufficient oxygen and a myocardial infarction or heart attack can occur. Atherosclerosis can occur throughout the human body, however, it is most life threatening within the coronary vasculature.
Transluminal angioplasty surgery utilizes an elongated, flexible catheter having an inflatable balloon at its distal end that is inserted at an appropriate position into the vascular system of a patient. After the catheter is inserted into the vascular system, its balloon is routed to a stenosis. Once the balloon is properly positioned relative to the stenotic lesion, it is inflated with fluid under relatively high pressure. As the balloon expands, it dilates the stenosis, thus allowing blood to flow more freely.
Ultrasonic imaging devices have been developed for imaging inner walls or inner peripheral features of a blood vessel so as to determine the location of a stenotic lesion or stenosis and to obtain a visual image of the stenosis for diagnosis purposes. An example of such an ultrasonic imaging device is disclosed in U.S. Pat. No. 4,917,097 issued to Proudian et al., which is hereby incorporated by reference.
Ultrasonic imaging devices such as the one illustrated in the '097 patent to Proudian et al. include piezoelectric elements or transducers for generating ultrasonic waves and detecting echoes or reflections off the inner wall of the stenotic lesion. The piezoelectric elements flex in response to a received electric pulse and generate an ultrasonic wave in response to the electrically-induced flexing. Mechanical relaxation of a piezoelectric element after it has been electrically excited results in a damped oscillation of the transducer element, which causes the element to generate an electrical signal typically referred to as a "ringdown" signal.
Initially, the ringdown signal generated by the piezoelectric element is generally a much stronger signal than the signal typically generated by an echo of the ultrasonic wave. In fact, the ringdown signal can be as much as 80 decibels (dB) larger than the echo signal. Because the ringdown signal is so large relative to the echo signal, the amplitude of the ringdown signal is enough to saturate the front-end amplifiers of the imaging device circuitry and thus create artifacts in the image. This saturation of the amplifiers effectively creates a blind spot or corona in the generated image corresponding to an area immediately adjacent to the surface of the transducers.
Interference with the echo signal by the ringdown signal has resulted in various attempts to solve the problem. One current method of removing the ringdown signal is to store a reference waveform that corresponds to the ringdown signal and other factors, such as the ambient environment, that create a characteristic damped mechanical oscillation pattern of each transducer when flexed in response to an electrical pulse. This characteristic reference waveform is stored and then later subtracted from received echo signals. Since a received echo signal includes both the reflected ultrasonic wave signal and the reference signal, subtracting the stored reference signal from the total received echo signal, or real-time signal, will theoretically leave only the reflected ultrasonic wave signal or imaging signal. This technique is effective in the region of the ringdown signal within a linear region of amplification; however, it is not effective in the region of the signal that is clipped by a saturated amplifier. Because the ringdown signal is initially so much greater in amplitude than the echo signal, the first part of the ringdown signal typically saturates the amplifiers of the imaging device at its highest amplitudes.
In order to obtain an accurate reference waveform corresponding to the ringdown signal, the imaging apparatus must be in an echo-free environment so that the received and stored signal is composed of only the reference waveform and not any reflections off the inner wall of the vascular system. The transducers of the imaging probe, which transmit the pulse signals and receive the echo signals, must perceive the same acoustic impedance during the collection of a reference waveform and during the collection of an echo signal so the reference signal is matched in phase and amplitude with the ringdown signal generated while imaging the blood vessel.
One technique for obtaining a reference waveform is to place the imaging device within a large water-filled tank before the catheter is inserted into the body of the patient. This technique makes it difficult to match the acoustic impedance of the water with that of the blood in the vascular system. Therefore, the amplitude and phase of the ringdown signal generated and recorded in the environment of the water-filled tank may be somewhat different than the ringdown signal generated in the blood. Another problem associated with this technique is drifting of the ringdown signal caused by variations in temperature between the water in the tank and the blood in the vascular system. Moreover, sterility of the catheter may also be compromised by placing the sterile catheter in a tank of water or saline solution prior to insertion into a patient.